1. Field of the Invention
This invention relates in general to intercepting service for party lines in terminal party line systems for telephone systems and in particular to a novel apparatus and method.
2. Description of the Prior Art
Intercepting service for automatic telephone systems has been available for many years. In terminal per station systems, the method of intercepting calls to stations on any line, individual or party, is relatively simple and straightforward since each main station is associated with its own individual connector terminal.
The method employed in terminal per line systems is, however, more complex because one connector terminal connected to a party line may be associated with several stations. The problem then is to find a method of distinguishing one particular station from the others. There have been a number of schemes employed depending upon the type of ringing supply used. The most common form of ringing in independent practice has been some form of multifrequency selective ringing.
Generally speaking, the last digit of the directory number designates the ringing frequency employed. Party line stations on the same line associated with the same connector terminal may have any digit from "1" to "0" as the last digit of the directory numbers. Since the ringing sources are limited to five frequencies, parties "1" to "5" have telephone ringers connected from one side of the line to ground while parties "6" to "0" have ringers connected from the opposite side of the line to ground. Thus, ten party selective ringing can be provided. Semi-selective ringing using ten bridged ringers can be obtained by using one or two ring signals of each of the five frequencies available.
There are other methods of ringing telephones such as non-selective code ringing using a single ringing frequency, superimposed ringing using a single frequency upon which is a superimposed a negative or positive battery potential to ground to achieve selectively of ringing for four parties. This system may also employ two rings of each type of superimposed ringing current to achieve semi-selective signaling on party lines having eight parties or three rings for parties 9 and 10 on a ten party line.
An early system of intercepting parties in terminal per line systems using multifrequency ringing used a modified traffic ringer on party lines. A traffic ringer comprises a telephone ringer movement tuned to ring on only one frequency. Instead of using conventional clapper and bells associated with the ringer, the traffic ringer was equipped with spring contacts which would close a circuit to an auxiliary relay which in turn would connect the associated line to a trunk. A traffic ringer in the central office was provided for each intercepted station on a party line. If more than one station on a line required intercept service, a traffic ringer was supplied for each station. Each traffic ringer would respond to only one frequency so while the connector terminal was jumpered to the intercept rack, only those stations requiring intercepts would be picked-up when the ringing frequency associated with the station was applied to the line. Active stations would not be effected. This practice has not been economical for general use although it is still used in many places. The use of the traffic ringer technique has lead to considerations that any intercepting scheme which is associated with a multifrequency ringing scheme should employ some form of frequency discriminating device for detection of a station to be intercepted.
The fundamental concept of match pulse intercept is based on the fact that at any given time only one frequency of ringing current is being supplied to a particular group of connectors. With matched pulse intercept, a pulse of positive battery is supplied to the intercept rack at the same time. This "matching" pulse is used to control intercepting line relays which will in turn connect an individual station on a party line to the intercept trunk.
Each of the foregoing ringing schemes present their own problems for intercepting device service and each has a particular solution. It should be realized that matched pulse apparatus is applicable to all of the ringing schemes presently in use.
Because harmonic or multifrequency ringing is quite common, in the independent field, this form of ringing will be described relative to the present invention.